Answered my own question with a bit more digging.
The astdb.txt file is consumed by server.php which expects to find it in /var/www/html/mobile. The file was there and the extended information for node 561485 was in it, but Allmon [1] was only showing the node number. I fixed this by simply restarting the asterisk service. Which is weird, because server.php seems to be called every few seconds and there are no apparent caching mechanisms which would prevent date being taken directly from astdb.txt and sent back to the client. Most unusual.
user@host:/var/www/html/allmon2$ grep -i 56148 astdb.txt
561480|GD5MUP|[RPG]|Douglas, Isle of Man
561481|GD5MUP|[VOV]|Douglas, Isle of Man
561482|GD5MUP|[DVSM cloud]|Douglas, Isle of Man
561483|GD5MUP|[MacBook Pro]|Douglas, Isle of Man
561484|GD5MUP|[ASL app]|Douglas, Isle of Man
561485|MB7IOM|144.96250|Douglas, Isle of Man
And the response from server.php:
event: nodes
data: {
"561482": {
"node":"561482",
"info":"GD5MUP [DVSM cloud] Douglas, Isle of Man",
"remote_nodes":[{
"node":"41522",
"info":"M0HOY HUBNet Manchester, UK",
"link":"ESTABLISHED",
"ip":"193.150.14.4",
"direction":"OUT",
"keyed":"no",
"mode":"T",
"elapsed":" ",
"last_keyed":" "
},{
"node":"561480",
"info":"GD5MUP [RPG] Douglas, Isle of Man",
"link":"ESTABLISHED",
"ip":"xxx.xxx.xxx.xxx",
"direction":"IN",
"keyed":"no",
"mode":"T",
"elapsed":" ",
"last_keyed":" "
},{
"node":"561485",
"info":" ",
"link":"ESTABLISHED",
"ip":"xxx.xxx.xxx.xxx",
"direction":"IN",
"keyed":"no",
"mode":"T",
"elapsed":" ",
"last_keyed":" "
}]
}
}
My sincere thanks for your assistance Mike, please forgive the wild goose chase.
Mark.
[1] Well, not Allmon, but a mobile-optimised UI of my own creation. 
